We have completed a detailed, high resolution cytogenetic study of bone marrow samples from 48 consecutive patients with acute nonlymphocytic leukemia (ANLL), using our 400-850 band methotrexate synchronization technique. In 45 patients, including 28 who were untreated, adequate mitoses were obtained. Because most leukemias studied were found to have a chromosomal abnormality (43 of 45 cases), it became important to determine whether a high percentage of the cases showed consistent defects and how they grouped in relation to defined ANLL subgroups. Significantly, of the 43 cases found to have a clonal chromosomal abnormality, approximately two-thirds showed one of seven recurrent defects. Among them, one-third of the patients (15 of 43) had either a deletion 5q, deletion 7q or trisomy 8. Also, one fourth of the patients (11 of 43) showed a chromosomal translocation 9;22, 8;21, 15;17 or a translocation between a chromosome 11 and a chromosome 6, 9 or 17. The remaining patients (17) showed random chromosomal abnormalities. Our results contrast with those carried in the seventies when it was believed that only 50% of all cases of ANLL had abnormal karyotypes and only a small fraction of these showed recurrent defects. The main reasons for the discrepancy between our results and those of the previous literature relate to two main points: (1) Our use of methotrexate cell synchronization of short-term cultured cells helped leukemic cells to selectively divide in vitro, while direct preparations may have shown a predominance of dividing erythroblasts. (2) The use of a very brief exposure of cells to colcemid to obtain elongated mitoses made it possible to uncover minute chromosome defects not previously detected. This is illustrated with our observation that a subgroup of patients with acute monocytic and acute myelomonocytic leukemia show a previously undetectable translocation involving chromosome 11 with breakpoint at band q23 with either a chromosome 6, 9, or 17. Specific objectives for the coming year are: (1) to study bone marrows from approximately 40 adult patients with acute nonlymphocytic leukemia utilizing methotrexate cell synchronization for the study of chromosomes at the 550, 850 and 1,200 band stages; and (2) to continue performing sequential studies of patients first studied at diagnosis and later during remission and relapse, to monitor secondary chromosomal changes and their relation to prognosis.